Add static assertion that uint32_t is unsigned int or wider

Summary:
This is a backport of secp256k1 [[bitcoin-core/secp256k1#818 | PR818]]

Depends on D7632

Test Plan:
  ninja check-secp256k1

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7635

src/assumptions.h

@@ -7,6 +7,8 @@
 #ifndef SECP256K1_ASSUMPTIONS_H
 #define SECP256K1_ASSUMPTIONS_H
 
+#include <limits.h>
+
 #include "util.h"
 
 /* This library, like most software, relies on a number of compiler implementation defined (but not undefined)
@@ -19,7 +21,11 @@ struct secp256k1_assumption_checker {
        allowed. */
     int dummy_array[(
         /* Bytes are 8 bits. */
-        CHAR_BIT == 8 &&
+        (CHAR_BIT == 8) &&
+
+        /* No integer promotion for uint32_t. This ensures that we can multiply uintXX_t values where XX >= 32
+           without signed overflow, which would be undefined behaviour. */
+        (UINT_MAX <= UINT32_MAX) &&
 
         /* Conversions from unsigned to signed outside of the bounds of the signed type are
            implementation-defined. Verify that they function as reinterpreting the lower